The present invention relates to an apparatus for evaluating the quality of rice grains and is more particularly concerned with an apparatus for evaluating the quality of rice grains based on the measurements of the content percentages of pre-selected constituent(s) of rice grains.
The quality of rice grains, particularly in view of the taste when cooked, varies due to many different factors such as those relating to rice production, that is, selection of rice plant varieties, production districts, cultivation methods, havesting methods, etc., those relating to rice grain processing after harvesting, that is, drying, storing, milling, etc., and those relating to the cooking of rice. As to the quality of rice grains, particularly from the view point of taste, those of the above relating to rice production have the most significant effect followed by those relating to rice grain processing.
Conventionally, the quality evaluation of rice grains, especially from the sense of taste of cooked rice, relied upon a personal examination and judgement according to an individual senses. The personal judgement means one in which a number of panel examiners repeatedly examine and evaluate the appearance, smell, taste, chewability, hardness, etc. of the sample grains to determine whether those being examined are superior or inferior as compared with reference grains for evaluation and the evaluation values thus obtained are averaged. However, as this personal judgement is effected based on a sense of taste which differs from person to person, the evaluation results obtained through the a personal judgement can in no way be considered an objective and absolute value invariable regardless of time and place. Research has been developed which is intended to measure and analyze the constituents and the physicochemical properties of the rice grains for pursuing the correlation between the values thus measured and analyzed and the evaluation result obtained through personal judgement so that the quality of the rice grains may be evaluated on a scientific basis. As a consequence, it was found that the content ratio between amylose and amylopectin which constitute the starch content, content percentages of protein and content percentages of moisture are particularly important factors for purposes of evaluating the quality of rice grains.
Accordingly, how the difference of content percentages of various constituents of rice grains affect the quality, especially in terms of taste of rice grains after having been cooked, is discussed in detail hereunder.
Generally, the varieties of rice which are most popular in Japan from the view point of taste are "Koshihikari" and "Sasanishiki". As an example, the content percentages of protein and the content ratio of amylose in 100% of the starch content of averagely milled white white of several kinds of rice varieties including the above "Koshihikari" and "Sasanishiki" are given in the following Table 1. It is, of course, needless to say that the same variety of rice does not necessarily contain the same content percentages of constituents as such content percentages become varied depending on the geological conditions (the soil and the water conditions) of the rice production districts and also on the atmospheric conditions (temperature, hours of sunshine, rainfall, etc).
TABLE 1 ______________________________________ PRODUCTION CONSTITUENTS (%) VARIETIES DISTRICTS Protein Amylose ______________________________________ Koshihikari Niigata 6.70 19.9 Sasanishiki Yamagata 6.89 20.9 Nihonbare Shiga 7.19 21.4 Ishikari Hokkaido 8.48 23.2 ______________________________________ (In the Table, the content percentages of protein are shown by weight ratio while the content percentages of amylose are shown by ratio in 100% of the starch content.)
From the above Table 1, it can be understood that the main factor accountable for the superiority of the taste of "Koshihikari" and "Sasanishiki" as compared with that of other resides in the fact that both the content percentages of the protein and the content ratio of the amylose in the starch are lesser as compared with those of other rice.
Besides the fact that the content percentages of protein and the content ratio of amylose in the starch significantly affect the taste of rice, that is, the quality of rice grains, the moisture content in the white rice also significantly affects the quality of rice, especially in relation to the chewability and hardness of cooked rice. When the moisture content of white rice is in the order of 15%, the rice grains do not develop cracks upon being immersed in water in a cooking pot so that the cooked rice remains in the best state. However, if the moisture content of the white rice is less than 14%, the rice grains are cracked instantaneously when they are immersed in water because of their rapid absorption of water. As soon as cracks develop in and through the rice grains, water is absorbed into such cracks. Broken rice grains also absorb water in the same way. When such rice grains are boiled, paste exudes out of the cracks and the boiled rice becomes watery, crumbly and too soft without any chewability. This problem of the content percentage of moisture of white rice grains becoming less than 14% may be mainly due to excess drying at the grain processing stage after harvesting, especially at the stage of drying operation, and also occurrence of cracking and heating due to friction during milling operation after the grain processing stage. Therefore, in order to avoid the quality of rice grains becoming lowered due to the content percentages of moisture being less than 14%, it is necessary that during drying process the drying machine be carefully controlled so that the grains do not become too dried and also that during milling process the milling machines be checked and regulated so that the rice grains are not broken due to the wearing out of the parts of the milling machines and there occurs no excess drying due to generation of heat. For the purposes of restoring the quality of rice grains whose content percentages of moisture have lowered to less than 14% due to such a reason as excess drying, there is commercially available a rice moistening machine. This machine is adapted to supply moisture to the white rice grains at a speed at which grains naturally absorb water or moisture so that the content percentages of moisture thereof is regulated up to around 15% which is the safe range for avoiding cracking in water.
In addition to the above explained content percentages of protein, starch and moisture of the rice grains which significantly affect the quality of rice grains, the content percentages of fat also affect the taste or quality of rice and it is usually said that lesser the content percentages of fat the better the taste. However, it can be said that the effect on the taste of the fat constituent is not so great as compared with that by the above three constituents.
Generally, since it is difficult to secure a single variety of rice grains of fine quality in a large amount for processing at a rice mill plant, rice grains of a number of varieties or a number of classes which are different in quality, for example, the rice grains believed to be of high grade and those believed to be of low grade, are blended with the blending ratio being appropriately controlled in an effort to produce a stable quality of white rice for distribution. However, the selection of rice varieties to be blended and the blending ratio therefor have been determined in the past based on personal judgement and data concerning the quality. In the absence of any methods that are supported from a scientific view point, the blended white rice thus produced for distribution has often tended to be irregular in quality and this has often been the subject of complaints by consumers.
On the other hand, as is conventionally well known and as is traditionally said, the taste of cooked rice is improved when ordinary rice (non-glutinous white rice) is added with a small amount of glutinous rice before cooking it and this is due to an increase in chewability. The following is a reason for this which is explained in relation to the changes in the content of chemical constituents of rice. The starch content consists of amylose and amylopectin and as is briefly explained hereinabove in connection with Table 1, W the taste of rice tends to be inferior if the content ratio of amylose in 100% of the starch content becomes large. If a small amount of glutinous rice whose content ratio of amylopectin is approximately 100% is mixed into an ordinary rice whose content ratio of the same is 78%, the taste of the mixed rice is improved to almost the same as the rice which contains a larger amount of content ratio of amylopectin, that is, the rice which contains a smaller amount of content ratio of amylose. However, if the content ratio of amylopectin exceeds an appropriate amount, the cooked rice becomes too viscous and this causes a deterioration of the taste of rice.
From the above discussions, it may be understood that ideas have arisen that the evaluation of rice quality can be done by having the chemical constituents of rice grains measured and analyzed scientifically, that the rice grains of good taste can be discovered from among ordinary varieties of rice grains without the need to rely on specific famous varieties of rice grains which are generally known to be good tasting rice, and further that the quality, especially from the view point of the sense of taste, of rice grains can be improved by the blending of a plurality of varieties of rice grains or blending of a plurality of rice grains whose content percentages of constituents are different from one another.